The maintenance of vehicles such as cars and trucks frequently requires access to the underside of the vehicles in order to permit repair of such parts as transmissions, clutches, gearing, joints, brakes, and the like. In order to reach these areas of a vehicle, a worker will typically employ one or more lifting devices that are positioned beneath the vehicle chassis or wheels and actuated to lift the vehicle above the ground.
Conventional lifting systems comprising a plurality of lifting devices may be powered by hydraulic or mechanical systems, which allow for a smooth raising and lowering motion throughout the range of travel as a result of small differences in static and dynamic friction within the system. Generally, the amount of force required to overcome static friction while a lift is at rest, is nearly the same as the force required to overcome the dynamic friction while the lift is in motion. However, single-acting gravity return cylinders in pneumatic lift systems have suffered from a great disparity in the forces required to overcome static and dynamic friction, as compared to their hydraulic and mechanical counterparts. The compressible nature of air results in the inability to obtain small and precise adjustments in order to maintain the smooth synchronization of lifts, resulting in more of a “ratcheting” motion of the lifts.
U.S. Pat. No. 5,484,134, which is herein incorporated by reference in its entirety, discloses pneumatic lifts for holding a vehicle in a lifted position while being worked on. U.S. Patent Application Publication No. 2013/0240812, which is herein incorporated by reference in its entirety, discloses a pneumatic lift system capable of performing an electronically synchronized lift using two or more individual lifts. The wheel lift system of the '812 application is pneumatically powered via an external source of compressed air, and the system is electronically controlled from a common control station/module. Although the lift system of the '812 application represents a significant advancement in automobile wheel lifts, the system of said application does not solve the problem of smoothly overcoming static and dynamic friction in pneumatic lift systems.